pinctrl: Add NVIDIA Tegra XUSB pad controller support

The XUSB pad controller found on NVIDIA Tegra SoCs provides several pads
that lanes can be assigned to in order to support a variety of interface
options: USB 2.0, USB 3.0, PCIe and SATA.

In addition to the pin controller used to assign lanes to pads two PHYs
are exposed to allow the bricks for PCIe and SATA to be powered up and
down by PCIe and SATA drivers.

Tested-by: Mikko Perttunen <mperttunen@nvidia.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>